^^^' 55'] GEOLOGY OP NOKTHERN ANGLESEY. 649 



shows some very interesting aud unmistakable examples of thrust- 

 planes. The section, which extends farther south than Prof. Blake's 

 figure, differs from his in some respects, but differences may well 

 be expected, for the section has to be constructed piecemeal as it 

 is slowly revealed by scrambles over the rocks. Parts of the 

 section are inaccessible, and some of the details are doubtful, but 

 in its main features it will be found correct. 



The Ordovician beds occupy a faulted syncline, overthrust on the 

 north, as Prof. Blake has pointed out, by limestone-beds. That the 

 fault is an overthrust one is rendered unmistakable by the series of 

 small thrust-planes which branch from the main fault, and clearly 

 indicate that the basal conglomerates have been moved in successive 

 steps across the succeeding sandy shales. The uppermost plane re- 

 presents a travel of about 15 feet, and the curling round of the broken 

 edges of the beds to lie in the plane of movement is well exhibited. 

 These conglomerates are seen in one place to lie on greenish gritty 

 phyllites which contain bands of fine grit and white quartzite. The 

 junction is undulating; there has certainly been some movement 

 along it, and the underlying rocks (N.A. 46 & 47) are highly crushed 

 and have become augen-sericitic schists or phyllites. These changes 

 seem, however, to have taken place to some extent before Ordovician 

 times, as pieces of sericite-schist occur in the conglomerates, and I 

 think that the relationship of the beds really represents an uncon- 

 formity. Moreover, at the head of the gulf the conglomerate lies 

 against a white quartzite : pebbles in the former appear to be 

 derived from the latter ; and the same relation between quartzite 

 -and quartz -conglomerate is to be seen at other localities in the 

 neighbourhood, as well as on the headland of Porth Llanlliana. 

 The conglomerates are largely made up of white quartz and quartzite- 

 pebbles, which closely resemble the ' quartz-knobs ' of the vicinity, 

 and they contain also occasional identifiable fragments of green slate, 

 limestone, and other rocks that occur in situ close by. 



Dr. Callaway found a similar assemblage in the conglomerates 

 fringing the boundary-thrust at Pyttiau, but there they contained 

 an abundance of limestone-blocks which corresponded exactly with 

 the Cemaes limestone. The phenomenon of Llandeilo limestone- 

 masses in the basal breccias at the western Porth Padrig must, how- 

 ever, make us very cautious in accepting the presence of calcareous 

 inclusions as proof of an unconformity. 



Perhaps, however, the most convincing proof is to be derived 

 from an examination of the geological structure of Mynydd-y-Garn, 

 a faulted inlier, mainly of contorted beds of the Green Series, lying 

 in the midst of Ordovician ground. The summit and the eastern part 

 of the hill are built up of a massive and crushed coarse conglomeratic 

 breccia, which passes up, through slates interbedded with breccia and 

 grit-bands, into the ordinary black shales of the Ordovician. This 

 massive breccia is the result of the denudation of granitic, gneissose, 

 and schistose rocks, granulites, grits, etc. : indeed, such an assemblage 

 as may be found in situ on Pen Bryn-yr-Eglwys, 2 miles distant. 

 The southern part of the inlier contains typical beds of the Green 



